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Local Deprotonation Enables Cation Exchange, Porosity Modulation and Tunable Adsorption Selectivity in a Metal-Organic Framework Jun-Hao Wang, Dong Luo, Mian Li, and Dan Li Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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Crystal Growth & Design

Local Deprotonation Enables Cation Exchange, Porosity Modulation and Tunable Adsorption Selectivity in a Metal-Organic Framework Jun-Hao Wang,†,§ Dong Luo,† Mian Li,† and Dan Li*,†,‡ †

Department of Chemistry and Key Laboratory for Preparation and Application of Ordered

Structural Materials of Guangdong Province, Shantou University, Guangdong 515063, P. R. China ‡

College of Chemistry and Materials Science, Jinan University, Guangzhou 510632, P. R. China §

Institute of Crystalline Materials, Shanxi University, Taiyuan 030006, P. R. China

ABSTRACT: This account demonstrates that under regulated synthetic conditions the protonated carboxyl sites in a neutral metal-organic framework (MOF), known as MOF-324 [formulated as Zn3OH(PzC)2(HPzC), H2PzC = 4-pyrazolecarboxylic acid], can undergo complete deprotonation, yielding NH4@ZnPzC [formulated as NH4⋅Zn3OH(PzC)3]. This modified, anionic framework with a pcu-g net is thus capable of postsynthetic cation exchange, which is highly modular, encompassing organic ammonium (Me3NH+, Et3NH+), main-group metal ions (Li+, Mg2+) and even lanthanide ions (Eu3+, Tb3+). The present approach is shown to be versatile and efficient in regulating porosity, fine-tuning gas adsorption property, and also endowing other functionality

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such as liquid-phase adsorptive separation (benzene/cyclohexane). In particular, the selective adsorption behaviors of CO2 over N2 in this system have been studied in detail, targeting optimized CO2/N2 selectivity, which are evaluated through uptake capacity, isosteric heat, and Ideal Adsorbed Solution Theory (IAST). Besides the competent CO2/N2 selectivity (e.g. 50.8 for Li@ZnPzC at initial CO2:N2 = 15:85), the current approach, when compared with the popular strategy of amine-grafting on exposed metal sites, shows the advantages of (i) modular porosity and adsorption property, (ii) facile exchange processes which are quantifiable and would not severely block the pore windows, and (iii) mild isosteric heat which is required for subsequent CO2 release. It is worthy to note the regulation of local deprotonation can be regarded as a general approach for MOF chemistry and functionalization, given that there are plenty of reported MOFs bearing such “open protonated sites”.

INTRODUCTION Metal-organic frameworks (MOFs),1 also known as porous coordination polymers (PCPs),2 are a promising class of hybrid crystalline porous materials3 with defined topologies4,5 and uniform pores, which exhibit widely spread applications as gas reservoirs,6-8 catalysts,9-11 separators,12-14 and sensors.15-17 Taking gas adsorption property, which is of utmost current interest due to energy and environmental concerns, as an example, MOF researchers have developed a variety of pre- and post-synthetic methods,18-20 aiming at enhancing the adsorptive ability and improving the separation performance in regard of carbon dioxide capture and storage.6,21-25 In terms of enhancing CO2/N2 adsorption selectivity, an intensely studied strategy was the use of open (or exposed) metal sites,6 notably demonstrated by Mg-MOF-7426 and CuBTTri.27 Such strategy can drastically enhance the adsorptive preference of CO2 over N2, an advantage that can be further strengthened via postsynthetic alkylamine-grafting. The resulting adsorbents, mmen-


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Mg2(dobpdc)28,29 and mmen-CuBTTri30 (mmen = N,N′-dimethylethylenediamine), are highly desirable, exhibiting high affinity with CO2 and exceptional CO2/N2 selectivity. However, this strategy still has some limitations. For example, the diffusion of amines into the interior pores is dependent on the binding strength of the amine to the metal site; those with strong affinity might block some of the pore windows, and thus severely reduce the adsorption ability.28 Therefore, the amines in use should be carefully selected, which means this grafting method is not modular in practice. An alternative approach for the regulation of adsorption affinity and selectivity of MOFs is through postsynthetic ion exchange.31-44 Compared with the above strategy of amine-grafting on open metal sites, there are several advantages for the cation exchange approach for modifying anionic MOF adsorbents. (i) Unlike the grafting methods which rely on specific interaction between the metal sites and amines, the exchanged guests here can be considered modular: all charged species, given suitable size, with different chemical nature can be implemented. Therefore, the pore volume/window size and adsorption behaviors can be systematically tuned. (ii) Cation exchange which relies on non-directional electrostatic interaction between the overall network and charged guests will facilitate the guest diffusion into the interior pores, and this process can be easily quantified. The prerequisite for utilizing the ion exchange approach is the MOF in question must have a charged framework.45,46 However, ionic MOFs are much rarer than neutral ones, and many existed ionic MOFs cannot maintain framework integrity during the ion exchange processes. An interesting example is two series of isoreticular (i.e. with same underlying topology) MOFs showing different charge states, namely, neutral [Ni3OH(L1)3(L2)1.5]⋅xguest (MCF-19 series)47,48 and cationic [In3OH(L1)3(L2)1.5](NO3) (ITC-n series).49 The present work reports the synthesis


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of an anionic ZnII MOF by introducing 1H-pyrazole-4-carboxylic acid (H2PzC) as the ligand under alkaline condition, yielding NH4⋅Zn3OH(PzC)3 (denoted as NH4@ZnPzC). We noticed an isostructural MOF, known as MOF-324 [formulated as Zn3OH(PzC)2(HPzC)],50 was obtained through a different synthetic procedure and having a neutral framework host. The complete deprotonation in the local building units has warranted cation exchange for NH4@ZnPzC, with the advantages of tunable adsorption behaviors and adjustable CO2/N2 selectivity. The cationic entities selected are of different chemical nature: organic ammonium (Me3NH+, Et3NH+), main-group metal ions (Li+, Mg2+) and trivalent lanthanide ions (Eu3+, Tb3+). The evaluations of CO2 and N2 adsorption behaviors include uptake capacity, isosteric heat, and Ideal Adsorbed Solution Theory (IAST) selectivity. To demonstrate the versatility of such a deprotonation-induced cation exchange approach, we also examine the performances of the cation-exchanged products towards liquid-phase adsorptive separation (benzene/cyclohexane).14 Because there are a number of neutral MOFs in literature bearing pendent, protonated carboxyl sites, one can propose the regulation of local deprotonation to be a general strategy for transforming neutral MOFs to anionic ones.

EXPERIMENTAL SECTION General Information. The ligand H2PzC (1H-pyrazole-4-carboxylic acid), other reagents and solvents were commercially available and used as received. Powder X-ray diffraction (PXRD) experiments were performed on a D8 Advance X-ray diffractometer using CuKα radiation (1.5418 Å). Infrared spectra were recorded on a Nicolet Avatar 360 FTIR spectrometer in the range of 3800-400 cm−1 on KBr pellets. Elemental analysis (C, H and N) was performed using a Vario EL III CHNS elemental analyzer. Thermogravimetric (TG) analysis was performed on a TA Instruments Q50 Thermogravimetric Analyzer under nitrogen flow of (40 mL·min−1) at a


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typical heating rate of 10 °C·min−1. 1H-NMR spectra were measured on a Bruker Spectrospin 300 spectrometer. For the determination of exchanged metal contents, the desolvated exchanged samples were digested in concentrated hydrochloric acid and measured through Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) on a Perkin-Elmer Optima-4300 DV instrument. Synthesis. Single crystals of NH4⋅Zn3OH(PzC)3 (hereafter referred to as NH4@ZnPzC) were grown from a triple-layered solution consisting of a methanol solution (2 mL) of H2PzC (0.1 mmol) as the top layer, DMF (2 mL) as the middle layer, and Zn(NO3)2 (0.1 mmol) in 25% ammonia solution (2 ml) as the bottom layer. After three months, the crystals were grown and isolated by filtration in 60% yield. The bulk samples of NH4@ZnPzC were obtained by adding the ammonia solution of Zn(NO3)2 and then the methanol solution of H2PzC into DMF (H2O:CH3OH:DMF = 1:1:4, V/V/V), and reacting under refluxing for about 6 hours. Refluxing time is crucial; longer than 10 hours would result in impurities. White powders of NH4@ZnPzC were obtained and collected after filtrating, washing with DMF and CH3OH for 3 times, respectively. The as-synthesized samples were activated with methanol (3 × 10 mL) over a three-day period before being dried (Yield 90%). Elemental analysis (%): Found: C 24.6, H 2.21 and N 16.85. Calcd: C 25.67, H 1.97 and N 17.46 for C12N7O7H11Zn3. IR (cm−1): 3375 (br), 3122 (w), 2970 (w), 2790 (w), 2476 (w), 1550 (s), 1450 (s), 1332 (m), 1287 (s), 1193 (w), 1041 (s), 1005 (s), 886 (m), 790 (s), 617 (m). Single-Crystal X-Ray Diffraction Study. Data collections of NH4@ZnPzC were performed on an Oxford Diffraction Gemini E (Enhance Cu X-Ray source, Kα, λ = 1.5418 Å) equipped with a graphite monochromator and ATLAS CCD detector (CrysAlis CCD, Oxford Diffraction Ltd) at room temperature. The data was processed using CrysAlis RED, Oxford Diffraction Ltd


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(Version 1.171.34.44, release 25-10-2010 CrysAlis171 .NET). The structure were solved by direct methods (SHELXTL-97) and refined on F2 using full-matrix last-squares (SHELXTL-97). All non-hydrogen atoms were refined with anisotropic thermal parameters, and all hydrogen atoms were refined with isotropic thermal parameters riding on those of the parent atoms. The PLATON SQUEEZES routine was applied to modify the reflection data because of the amount of disordered solvent present in the pores of the MOF. Parameters for data collection and refinement are summarized in Table S1 (see the Supporting Information). Selected bond lengths and angles for all complexes are given in Table S2 (see the Supporting Information). Cation Exchange Experiments. The extra-framework ammonium cations included in NH4@ZnPzC can be postsynthetically exchanged with Li+, Mg2+, La3+ (La = Eu3+, Tb3+), Me3NH+ and Et3NH+ cations, affording Li@ZnPzC, Mg@ZnPzC, La@ZnPzC, Me3NH@ZnPzC and Et3NH@ZnPzC, respectively. In a typical experiment, the sample of NH4@ZnPzC was suspended in methanolic solution of metal nitrate salts (e.g. LiNO3, Mg(NO3)2 and La(NO3)3) or organic amines (e.g. Me3N and Et3N, concentration 0.1-0.5 M) for 8 days, with corresponding methanol solution refreshed every four days. All the exchanged solids were subsequently filtered, washed with methanol for 3 times and later on suspended in methanol for 24 hours. After drying the exchanged samples under vacuum at 85 °C for 48 hours, the degree of cation exchange was determined by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICPAES) for metal ions and 1H-NMR analysis for organic amines. The data indicated that 100% of the ammonium cations were exchanged with Li+, Mg2+, La3+ (La = Eu3+, Tb3+), 85% for Et3NH+ and about 30% for Me3NH+ (see the Supporting Information for details). The unaltered PXRD patterns indicated the maintenance of the parent structure (see Figure 1c).


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Low-Pressure Gas Adsorption Measurements and Evaluations. The gas adsorptiondesorption experiments were performed on a Micromeritics ASAP 2020M surface area and pore size analyzer. All gases used were of 99.999% purity. Prior to the measurements, the samples were activated at 85 °C under vacuum according to the TG analysis (Figure S1 in the Supporting Information), and then treated by using the “outgas” function of the surface area analyzer for 6 hours at 120 °C. Adsorption isotherms for N2 were monitored at 77 and 273 K, respectively. CO2 adsorption isotherms were measured at 273, 298, 303 and 308 K, respectively. Surface area and pore size distribution were determined from the N2 gas isotherms at 77 K. Multipoint BET and the Langmuir surface areas were estimated by using the data recorded at P/P0 = 0.0001 - 0.1 atm. The pore size distribution were calculated from Saito-Foley method51,52 using data recorded at P/P0 < 0.6 atm. All the adsorption data used for calculation of selectivity and the isosteric heat (Qst) were fitted using dual-site Langmuir-Freundlich equations.6,21,22 The Clausius-Clapeyron equation was employed to calculate the enthalpies of CO2 adsorption.6,21,22 The selectivity of CO2 over N2 was evaluated using Ideal Adsorbed Solution Theory (IAST, see the Supporting Information).6,22,53 Detailed adsorption data and analysis methods are given in the Supporting Information.

RESULTS AND DISCUSSION Preparation and Deprotonation-Induced Cation Exchange. NH4@ZnPzC was prepared under alkaline condition. The anionic framework [formula Zn3OH(PzC)3-] contains ammonium cations in the channels (Figure 1a), which is different from MOF-324 [formula Zn3OH(PzC)2(HPzC)] with partially deprotonated carboxylic moieties and thus a neutral framework.50 The complete deprotonation of the carboxyl sites in NH4@ZnPzC is confirmed by the disappearance of the strong band at 1658 cm-1 in the infrared spectra (Figure 1b, inset and Figure S2 in the Supporting


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Information). Except for the existence of extra-framework NH4+ cations, the crystal structure of NH4@ZnPzC is almost identical to that of MOF-324, as indicated by PXRD patterns (Figure 1b).

Figure 1. (a) Synthesis of NH4@ZnPzC and schematic postsynthetic cation exchange process. (b) Comparison of PXRD patterns of NH4@ZnPzC (bulk samples) and MOF-324 (simulated from single crystal data). Inset: IR spectrum of NH4@ZnPzC. (c) Comparison of PXRD patterns of NH4@ZnPzC (simulated from single crystal data) and cation-exchanged samples.


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Figure 2. (a) Distorted cubic cage (pore diameter ca. 1.0 nm) viewed from of the diagonal direction of the unit cell of NH4@ZnPzC. (b) Top and (c) side views of the Zn3O SBU in NH4@ZnPzC, showing the multiple contacts between NH4+ and the SBU. Color codes: Zn, green polyhedra; O, red; N, blue; C, black, H, omitted. The NH4+ cation resides at the top of the Zn3O secondary building unit (SBU) and shows multiple contacts (dN3-O1 = 3.5625 Å; dN3-O3 = 2.7401 Å) with the COO- and OH- moieties in the SBUs, which are linked by the ligands into distorted cubic cages within the framework (Figure 2). The extra-framework NH4+ ions can be exchanged by other organic amine cations (Me3NH+ and Et3NH+) or metal ions (Li+, Mg2+, Eu3+, Tb3+ and mixed Eu3+/Tb3+). All the cationexchanged samples retained their crystalline integrity, as shown by PXRD patterns (Figure 1c).


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TG analysis (Figure S1 in the Supporting Information) indicated NH4@ZnPzC showed better thermal stability (decomposition temperature Td = 400 °C) than MOF-324 (Td = 250 °C).50 This is perhaps due to the existence of above-mentioned multiple contacts between NH4+ and the SBU. In comparison, Me3NH@ZnPzC and Et3NH@ZnPzC showed lower temperature to lose Me3N and Etz3N (150 °C and 230 °C, respectively), corresponding with the lack of hydrogen-bondingalike contacts. Mg@ZnPzC (Td = 440 °C) showed better thermal stability than that of the Li+exchanged sample (Td = 390 °C), which is perhaps due to the higher valency of Mg2+ that results in stronger electrostatic interaction with the MOF host. The exchanged samples can retain their crystallinity after thermal activation at 85 °C under vacuum for 48 hours (Figure S3 in the Supporting Information). However, they are sensitive to boiling water, as revealed by the new diffraction peaks in PXRD patterns (Figure S4 in the Supporting Information). Topological Analysis. We note that MOF-324 was described to have the same topology as MOF-554 with a primitive cubic (pcu) net,55 since each SBU is connected to six neighboring SBUs through PzC/HPzC linkers.50 However, the two structures differ at least in two notable aspects: (i) the composition and geometric shapes of the SBUs are distinct (octahedral Zn4O SBU for MOF-5, chair-like Zn3O SBU for MOF-324 and NH4@ZnPzC, see Figure 3); (ii) their crystal structures has different cubic space groups (MOF-5 crystallized in Fm-3m whereas MOF324 and NH4@ZnPzC in Pa-3; note pcu has symmetry Pm-3m). According to a consistent approach for MOF topological analysis,4,5,56 here we call attention to the essence of such an approach, in which one is supposed to consider linking geometric shapes to form the underlying nets; that is to say, the augmented net (e.g. pcu-a, also known as cab) is a better than its basic net (e.g. pcu) for describing the structures of MOFs.


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Figure 3. Comparison of the M4O SBUs (M = Zn or Co) in three reported MOFs and Zn3O SBU in NH4@ZnPzC, as well as their simplified geometric shapes and underlying nets. Figure 3 illustrates the procedure for deconstructing some related MOFs into their underlying nets. Similar to the carboxyl group, the pyrazolyl group can form Zn4O SBUs, which has been widely documented in literature for MOFs with single or mixed carboxylate/pyrazolate ligands, such as Zn4O(BDC)3 [MOF-5, BDC = 1,4-benzenedicarboxylate],54 Zn4O(BDC)(BPz)2 [Bpz = 3,3′,5,5′-tetramethyl-4,4′-bipyrazolate],57 Co4O(BDPz)3 [MFU-1, BDPz = 1,4-benzenebis(3,5dimethyl-pyrazolate)],58 and Zn4O(DMPzC)3 [DMPzC = 3,5,-dimethyl-4-pyrazolecarboxylate].59 It is unambiguous that these three M4O SBUs (M = Zn or Co) can be simplified into an


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octahedron in a topological viewpoint, and the corresponding net is pcu-a (i.e. cab).55,56 Now turning back to the Zn3O SBU with PzC ligands in this work, one can see that although each SBU is also linked to six neighboring ones, its geometric shape is more similar to a chair conformation other than an octahedron. By considering this chair shape in the simplification, the augmented net for NH4@ZnPzC is pcu-g.55 Note that pcu-g has the intrinsic symmetry of Ia-3, which is more compatible with that of the real structure of NH4@ZnPzC (space group Pa-3, compared with pcu-a in Pm-3m symmetry). To consider the augmented nets is useful in (i) describing the exact structural feature of MOF structures (shown above for the distinction of Zn4O and Zn3O SBUs), (ii) distinguishing MOF structures that are closely related but different in symmetry, and (iii) predicting new theoretical nets for the design of MOFs (many of this type are collected in RCSR).55 For (ii), another interesting example is [Zn3S(AmTAZ)3]⋅NO3⋅H2O (HAmTAZ = 3-amino-1,2,4-triazole),60 which exhibited a similar Zn3S SBU, but it has a cationic framework filled with charge-balancing nitrate anions. At first glance, the Zn3S SBU is also 6-coordinated and thus the framework has a pcu topology, but the framework has two types of embedded cages. By considering the SBU as the chair conformation as in NH4@ZnPzC, the augmented net turns out to be pbp,55 which retains the structural feature of two types of cages and can be distinguished from pcu-a and pcug. Concerning (iii), in relation to the cubic pcu-g (symmetry Pa-3), there is a predicted net pcu-h (hexagonal symmetry R-3m) in RCSR55 which is also based on the chair-like building unit and awaits to be discovered in a real MOF. Porosity Modulation. After thermal activation, the NH4@ZnPzC and cation-exchanged samples were subject to N2 adsorption at 77 K (Figure 4a). The profiles of the N2 sorption isotherms in this work differ significantly from that of MOF-324.50 All the samples showed


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Type-IV sorption behavior according to the IUPAC classification of physisorption isotherms,2,6 showing two-step adsorption and obvious hysteresis upon desorption. The first-step uptakes at lower pressures and subsequent horizontal plateau corresponding to type-I isotherms indicate the presence of micropores accessible to N2 molecules. The second-step uptakes at higher pressures show doubled uptakes for Et3NH@ZnPzC and Mg@ZnPzC and almost quadrupled uptakes for NH4@ZnPzC and the other three exchanged samples. Such a phenomenon is usually observed in mesoporous materials, indicating that during the crystal growth or postsynthetic modification processes there may be mesopores forming inside the crystals. However, further experiments are needed to validate this explanation. The pore size distribution plots (Figure 4b) indicate the porosity of the exchanged materials are properly regulated, and there are indeed a certain content of pores of other sizes forming in some of the samples (e.g. NH4@ZnPzC and Tb@ZnPzC).

Figure 4. (a) N2 adsorption isotherms of NH4@ZnPzC and cation-exchanged samples at 77 K (filled square: adsorption; open square: desorption). (b) Pore size distribution calculated from Saito-Foley method using N2 adsorption data at 77 K recorded at P/P0 < 0.6.


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Through postsynthetic cation exchange, the pore size and adsorption behaviors of the material can be properly modulated, although they showed reduced uptake capacity in contrast to MOF324 (ABET = 1600 m2/g)50 and other topical MOFs (see Table 1). The BET surface areas of NH4@ZnPzC and the cation-exchanged samples are all lower than 500 m2/g (listed in Table 1). Table 1. Comparison of Adsorption Performances of MOFs in This Work and Literature

MOFs

ABET (m2/g) a CO2 uptake (wt %) b Qst (kJ/mol) c CO2/N2 Selectivity d

NH4@ZnPzC

132.35

3.73

37.2-31.1

29.2

Me3NH@ZnPzC

97.33

4.06

40.6-22.2

43.2

Et3NH@ZnPzC

479.61

7.24

31.6-26.4

38.7

Li@ZnPzC

56.15

4.32

48.5-24.7

50.8

Mg@ZnPzC

359.25

5.29

32.4-21.1

21.9

Tb@ZnPzC

71.23

2.24

39.5-32.5

21.5

MOF-324 50

1600

/

/

/

Cu-BTTri 27

1770

14.3

21

21 e

en-CuBTTri 27

345

5.5

90

25 e

mmen-CuBTTri 30

870

15.4

96

327 e,f

Mg-MOF-74 22,26

1495

35.2

47

148.1 f,g

mmen-Mg2(dobpdc) 28

70

14.5

71

200 e,f,h

a

BET surface area. b CO2 uptake capacity obtained at 298 K and 1 atm. c Isosteric heat of adsorption for CO2 uptake. d IAST selectivity for 15:85 CO2:N2 at 273 K and 1atm. e Obtained at 298 K. f CO2:N2 15:75. g Obtained at 323 K. h Molar selectivity. Tunable CO2/N2 Adsorption Selectivity. We focused on examining the influence of cation exchange on the selective adsorption of CO2 over N2, targeting optimized CO2/N2 selectivity. The CO2 and N2 adsorption isotherms (273 K) for NH4@ZnPzC and the cation-exchanged samples are compared in Figure 5a. As expected, all the samples showed much higher uptake capacity for CO2 compared with N2, which is attributed to the greater quadrupole moment of


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CO2 facilitating its strong affinity with the extra-framework cations.6 Under ambient conditions (298 K, 1 atm), all exchanged samples showed greater uptake amounts of CO2 than that of NH4@ZnPzC (see Table 1). Among them Et3NH@ZnPzC has the highest CO2 uptake capacity (12.7 wt % at 273 K, 7.24 wt % at 298 K). The CO2 uptake capacity in this system has limited relation with the surface areas. For example, Li@ZnPzC has the lowest BET surface area. Nevertheless, it exhibited the third highest CO2 adsorption capacity, and its low-pressure uptake (< 0.09 atm) even exceeded that of Et3NH@ZnPzC, indicating the exchanged Li+ may endow the strongest affinity between CO2 and the adsorbents in this system.


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Figure 5. N2 and CO2 adsorption properties of NH4@ZnPzC and cation-exchanged samples. (a) Adsorption isotherms at 273 K. (b) Isosteric heats of CO2 adsorption calculated from isotherms at 298, 303 and 308 K. (c) IAST selectivity for a 15:85 CO2:N2 mixture at 273 K. To quantify the strength of the adsorbate-adsorbent interaction, we have measured the coverage-dependent isosteric heats of CO2 adsorption (Qst, Figure 5b and Table 1) by utilizing the Clausius-Clapeyron equation and the dual-site Langmuir-Freundlich fitting parameters (see the Supporting information for details).6,21,22 The Qst plots show great variation on the zerocoverage adsorption enthalpy and also its trend upon loading, indicating not only the framework porosity but also the strength of interaction sites is properly modulated in this system. Compared with NH4@ZnPzC, Me3NH@ZnPzC and Li@ZnPzC have higher zero-coverage Qst but the Qst values drop rapidly upon loading. The Qst behaviors for Et3NH@ZnPzC and Mg@ZnPzC are different: they exhibit slightly lower zero-coverage Qst and a milder changing trend upon loading CO2; also their CO2 uptake capacity are higher than the other samples. Such adsorption performance is favorable for practical application. The Qst values for Tb@ZnPzC is fluctuant, and it has the lowest uptake capacity; therefore, Tb@ZnPzC is considered the least applicable in the present system. In particular, the highest zero-coverage Qst (48.5 kJ/mol) for Li@ZnPzC is consistent with the above observation on CO2 uptakes. This is reasonable because (i) the exchanged metal content for Li@ZnPzC is about twice or trebly of that for Mg@ZnPzC and Tb@ZnPzC, respectively, due to charge balance, and (ii) Li+ has the smallest ionic radius of all exchanged cations and thus the strongest polarization ability to enhance the electrostatic potential interacting with CO2. Compared with several topical MOFs developed for CO2 capture (see Table 1), the zerocoverage Qst values (ranging from 31.5-48.5 kJ/mol) in this system can be considered appropriate


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for practical application, which are high enough for effective selective adsorption of CO2 over N2, but not as high as those of amine-grafting MOFs such as en-CuBTTri (90 kJ/mol), mmenCuBTTri (96 kJ/mol) and mmen-Mg2(dobpdc) (71 kJ/mol). This is beneficial for minimizing the energy penalty for regeneration.6 We then evaluate the CO2/N2 adsorption selectivity by applying IAST (see the Supporting information for details).6,22,53 The results are depicted in Figure 5c and compared with literature in Table 1. Among all the samples, Li@ZnPzC showed the highest CO2/N2 selectivity (67.5 at 0.1 bar, 50.8 at 1 bar) over the entire pressure range, in accord with the highest zero-coverage Qst mentioned above. The modulation of CO2/N2 selectivity through cation exchange is realized. Another material worthy of mentioning is Et3NH@ZnPzC, which combines the merits of competent CO2 adsorption capacity (12.7 wt % at 273 K, 7.24 wt % at 298 K), appropriate Qst values (31.6-26.4 kJ/mol) and considerable CO2/N2 selectivity (38.7 at 1 bar). More importantly, it exhibits almost constant CO2/N2 selectivity upon increasing the pressures (from 34 at 0.1 atm to 38 at 1 atm, see Figure 5c, green dots). Such a trend is also revealed by the milder variation of Qst values in the coverage range (compare the green dots with the other ones in Figure 5b), which is perhaps attributed to the narrowest pore size distribution (Figure 4b) for Et3NH@ZnPzC. This is advantageous in practical application because most separation processes are conducted under ambient pressure, whereas for most adsorbents the sorption selectivity would decrease sharply upon increasing the pressure to 1 atm. Liquid-Phase Separation of Benzene/Cyclohexane. To demonstrate the versatility of this cation-exchange modulation approach, we also examine liquid-phase separation of benzene over cyclohexane, which have close boiling points (80.1 and 80.7 °C, respectively) and sizes (dynamic diameter 5.8 and 6.0 Å, respectively).14 The activated samples of NH4@ZnPzC and cation-


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exchanged materials were immersed into 1:1 benzene/cyclohexane mixtures, and the resultants were acid-hydrolyzed and then analyzed through

1

H-NMR spectra (see the Supporting

information for details). The results indicated that NH4@ZnPzC, Li@ZnPzC, Mg@ZnPzC and Tb@ZnPzC showed negligible adsorption for benzene and cyclohexane. For Me3NH@ZnPzC, an enrichment of the adsorbed benzene over cyclohexane (3.1:1) was observed. The separation performance can be improved in the case of Et3NH@ZnPzC, for which the selectivity for benzene/cyclohexane reached 14:1. Such a result is in accord with the pore size distribution analysis (Figure 4b), in which Et3NH@ZnPzC has the smallest pore diameter and narrow distribution, therefore enabling size exclusion separation for benzene/cyclohexane. These proofof-concept experiments further substantiate the cation-exchange approach can also be used for tuning the adsorption selectivity in the liquid phase.

CONCLUSIONS In summary, we report here that under alkaline condition, an anionic porous framework NH4@ZnPzC can be prepared, which has the same topology (pcu-g net) as the neutral MOF-324 but differs in the extent of deprotonation of the local carboxyl sites (i.e. MOF-324 has “open protonated sites”). The complete deprotonation in NH4@ZnPzC, which is verified through IR spectra, TG analysis and gas adsorption isotherms, etc., has warranted the inclusion of NH4+ cations that are exchangeable with other organic ammonium (Me3NH+, Et3NH+), main-group metal ions (Li+, Mg2+) and even lanthanide ions (Eu3+, Tb3+). This deprotonation-induced cation exchange approach is demonstrated to be versatile and efficient in regulating porosity, fine-tuning gas adsorption selectivity, and enabling adsorptive separation in the liquid phase. Because there are many reported neutral MOFs bearing such “open protonated sites”, it is logical to propose that the regulation of local deprotonation to be a


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general strategy for transforming neutral MOFs to anionic ones, and can further endow wideranging applications through porosity modulation. In particular, NH4@ZnPzC and the cation-exchanged materials have been investigated toward selective adsorption of CO2 over N2. The present approach benefits from (i) modular porosity and adsorption property, (ii) facile exchange processes which are quantifiable and would not severely block the pore windows, and (iii) mild isosteric heat which is required for subsequent CO2 release, when compared with the popular amine-grafting strategy. Among the materials studied, Li@ZnPzC shows the highest zero-coverage heat of adsorption (48.5 kJ/mol) and CO2/N2 selectivity (67.5 at 0.1 bar, 50.8 at 1 bar), while Et3NH@ZnPzC combines the merits of competent CO2 uptake capacity (12.7 wt % at 273 K, 7.24 wt % at 298 K), mild and constant Qst values (31.6-26.4 kJ/mol) and considerable CO2/N2 selectivity (38.7 at 1 bar). Therefore, we conclude the aim of optimizing CO2/N2 selectivity through cation exchange has been realized, with Et3NH@ZnPzC being the most suitable material for CO2/N2 separation application in this system.

NOTE ADDED AFTER SUBMISSION After the original submission, we found that, besides Zn3OH(PzC)2(HPzC) (i.e. MOF-324),50 there were two structures reported previously, namely Zn3OH(PzC)1.5(OH)(H2O)3.5⋅(PzC)0.561 and H3O⋅Zn3OH(PzC)3 (FIR-51),62 which are related to NH4⋅Zn3OH(PzC)3 (i.e. NH4@ZnPzC). These previous studies did not focus on the postsynthetic cation exchange and subsequent modulation of gas adsorption properties.

ASSOCIATED CONTENT Supporting Information


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Experimental section, crystal data (CCDC no. 1533674), additional physical measurements and gas adsorption data and analysis. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Basic Research Program of China (973 Program, Nos. 2012CB821706 and 2013CB834803), the National Natural Science Foundation of China (Nos. 91222202 and 21171114). REFERENCES (1) Furukawa, H.; Cordova, K. E.; O’Keeffe, M.; Yaghi, O. M. Science 2013, 341, 1230444. (2) Kitagawa, S.; Kitaura, R.; Noro, S.-i. Angew. Chem. Int. Ed. 2004, 43, 2334–2375. (3) Férey, G. Chem. Soc. Rev. 2008, 37, 191–214. (4) Li, M.; Li, D.; O’Keeffe, M.; Yaghi, O. M. Chem. Rev. 2014, 114, 1343–1370. (5) Schoedel, A.; Li, M.; Li, D.; O’Keeffe, M.; Yaghi, O. M. Chem. Rev. 2016, 116, 12466– 12535.


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Synopsis for Table of Contents (TOC)

Deprotonation-induced cation exchange has enabled modulation of porosity and selective adsorption, as well as optimization of CO2/N2 separation performance in NH4@ZnPzC system.


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Local Deprotonation Enables Cation Exchange ... - ACS Publications

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